The present invention relates to a color facsimile signal transmission system with an extremely simple circuit construction which is capable of transmitting color signals as well as black-and-white facsimile signals.
Facsimile signal transmission in this country is divided into audio frequency band transmission which utilizes the cleared channel of the Nippon Telegraph & Telephone Public Corporation and transmission which utilizes a cleared channel of a wide band of 12 to 240 KHz. The subscription facsimile utilizing the public telephone exchange network is divided into a system wherein transmission is effected by switching the peripheral circuit from a telephone receiver to a facsimile device, and a system wherein a facsimile signal is acoustically coupled to the transmitter and the receiver of a telephone.
The facsimile using the telephone circuit utilizes, in general, transmission systems of DSB (double side band wave) of AM and VSB (vestigial side band wave). For high speed transmission, various systems have been proposed such as a system wherein a binary black-and-white facsimile signal is converted into a three-value signal, or a system wherein the lengths of white and black components (run lengths) of a facsimile signal are encoded along with adjacent scanning lines for transmission.
In a color facsimile system, an original color image is resolved into three colors for transmission. For use of the image for an original plate for color printing at the receiving side, the three colors are separately received on black-and-white film. For use as an original image for color television broadcasting or for color photography, the three colors are combined and recorded on media such as color film.
As a conventional system for transmitting color facsimile signals, a line sequential system has been proposed wherein color information for each image element of a transmitting image is resolved into three primary colors, and signals representative of these three primary colors are sequentially transmitted in each scanning line. However, with this system, the transmission time corresponding to each scanning line is three times the usual case. Further, these signals cannot be transmitted in a compatible manner with usual black-and-white facsimile signals.
Another conventional system has been proposed wherein a code for differentiating colors is added to the initial part of each image element. However, with this system, the transmission speed is also slow as compared with usual black-and-white facsimile signals. Further, these signals cannot be transmitted in a compatible manner with usual black-and-white facsimile signals.
Another transmission system is known wherein the polarities of the black-and-white signal and the color signal are opposite, and the signals are transmitted in a three-value form (Japanese Patent Publication No. 52-31607). In this system, a phase synchronizing signal (FIG. 1(a)), a first color signal (FIG. 1(b)), and a signal (FIG. 1(d)) derived by inverting a second color signal are superposed at the transmitting side to obtain an output three-value signal of +1, 0 and -1. This output signal is amplified and transmitted to the receiving side. The receiving side separates the first and second color signals depending on whether the output of an entire-phase envelope detector is "1" or "0".
However, since one polarity of the three-value signal is used for the color signal, the black-and-white facsimile signal becomes essentially the same as a binary signal. Thus, the transmission time becomes longer as compared with the case of a general three-value signal.
Further, this system has been defective in that it cannot be used with the AM three-value system designed for black-and-white signals.